Solid ink or phase change ink printers conventionally receive ink in a solid form, either as pellets or as ink sticks. The solid ink pellets or ink sticks are typically inserted through an opening of an ink loader for the printer, and the ink sticks are pushed along the feed channel by a feed mechanism and/or move under the effects of gravity toward a heater plate in a heater assembly. The heater plate melts the solid ink impinging on the plate into a liquid that is delivered to a melt reservoir. The melt reservoir is configured to maintain a quantity of melted ink in liquid or melted form and to communicate the melted ink to a reservoir in one or more printheads as needed.
Within the printheads, heaters maintain the ink in the printhead reservoirs and jetstacks in liquid form. These heaters are usually energized with AC power from the 115/230 VAC RMS mains of a facility's power grid. The AC power is regulated using semiconductor triac switches. Because the heaters are connected to the input AC power mains, they typically meet UL, CSA, and manufacturer safety requirements for construction. In the event of a fault condition, manufacturers typically require that the heater construction be able to pass an appropriate safety standard, such as a 1,500 VRMS hi-pot withstand test for a single insulated constructed heater or a 3,000 VRMS hi-pot withstand test for a double insulated constructed heater, for a one-minute interval even after a “thermal runaway” fault condition. Thermal runaway is described as the loss of input AC power regulation that results in AC power being continuously applied to the heaters. The loss of input AC power regulation normally occurs in response to a failed semiconductor triac switch shorting in a manner that directly couples AC power to the heater. The continuous application of input power causes the heaters to heat until they either burn open or an in-line thermal fuse disconnects the AC power from the heaters.
The in-line thermal fuses address the thermal runaway condition by sensing the heater temperature and disconnecting the input power from the heater in response to the heater temperature rising above the threshold temperature of the fuse. The decoupling of the input power from the heater helps avoid damage to the heater. Manufacturers typically require that a heater be able to pass one of the withstand tests after a thermal runaway event. In order to achieve this goal, the thermal fuse should respond before the ability of the heater to pass the withstand test is degraded. Providing timely responses to thermal runaway events is a desirable goal in solid ink printers.